Arms control treaties up to and including Strategic Arms Reduction Treaties (START) have used the approach of counting nuclear weapon delivery systems as the means of controlling the number of weapons. They have concentrated on deployed weapons and not stored weapons. With future efforts to be directed at actually counting the number of nuclear weapons and reducing the number of weapons, the long-term goal of arms control efforts becomes the same as one of the goals of the nuclear nonproliferation initiative: the reduction of current weapons stockpiles and special nuclear materials (SNM). This means that nuclear weapons must be disassembled and the SNM rendered useless for a weapon to be considered destroyed. This can be accomplished by either changing its shape and size (crushing, cutting) or fabricating it into a reactor fuel and burning it in a reactor.
Once the counting of nuclear weapons, as opposed to counting delivery vehicles or re-entry vehicles, for the purpose of dismantlement is begun, some method is necessary to determine that the item to be dismantled is a nuclear weapon. In addition, there must be some means to verify that the material in the item being examined before or after dismantlement, is SNM, what specific SNM is present, and that the SNM inside the weapon has been completely removed during disassembly. It is also necessary to know that all of this material is entered into the tracking system for accountability. This must be accomplished without weapons design information being revealed to anyone involved in this procedure.
The traditional safeguards techniques are far too intrusive to be used at this initial stage when the SNM is in a weapon configuration. This is true for a variety of reasons, but there is also the point that safeguards techniques often rely on well-calibrated equipment for specific configurations of SNM. This can present a difficulty for a field measurement. For example, when a weapon is removed from a missile, airplane, or storage bunker, the SNM is often surrounded by quantities of various materials that act as shielding or sources of additional radiation that are unknown to the verification inspector. Safeguards techniques would not give a correct analysis unless this configuration were known in detail. Such knowledge of the configuration would be considered restricted design information. These initial measurements would also have to be done in the field, when the weapon is first removed from the delivery system. This would be done possible in an extreme environment and, probably, after lengthy travel by the operating personnel and transportation of the equipment. Delicate equipment requiring precision calibration and stability for use might not work well in such a situation.
A field system has been developed to perform this scenario. It is called the Gamma Neutron Assay Technique (GNAT), and it uses neutron and high purity (HP) Ge gamma-ray detectors with coincidence requirements. Most importantly, it uses the physics of the fission process to specify what is required to be measured.
Such a system of multiple detectors operated in a prompt coincidence mode also has applications for identification of specific isotopes in a high gamma ray and/or neutron radiation field. It could be used to assay high-level fissile waste. The prompt coincidence mode can be described as declaring a nuclear fission event as having occurred if two or more pulses from two or more gamma-ray detectors, or two pulses from two or more neutron detectors, or one or more gamma rays and one or more neutron pulses occur within picoseconds (10.sup.-12) of each other.
It is also possible to use a gamma-ray detector array without use of the neutron detectors for certain types of fissionable materials other than weapons.